The present invention relates to a cemented carbide useful as a material for drills whose surface is coated with a coating of hard carbon such as diamond, non-crystalline diamond etc. and which are used for forming small diameter holes in printed circuit boards (hereafter referred to as PCB drills); and to the above-described kind of PCB drill itself. The cemented carbide to which the present invention is directed can be one in which the component which forms the binder phase is not limited to Co but may also be another metal from the iron group such as nickel. However, the present invention shall be hereunder explained taking Co as a representative example of the component which forms the binder phase.
Printed circuit boards are made by binding glass fibers with epoxy resin and then attaching a film of copper on the surface thereof. In recent years, printed circuit boards are being made with higher and higher density and consisting of more and more layers. Accordingly, there has been a demand that a material for a PCB drill to drill small diameter holes has wear resistance and breakage resistance significantly higher than conventional materials.
WC-Co cemented carbides are generally used as materials for PCB drills. When WC-Co cemented carbides are used as the material for the drill, the hardness can be increased without decreasing the toughness, by reducing the size of the WC grains. From this standpoint, the cemented carbides disclosed in Japanese Patent Application Publication No. Sho 61-12847 and Japanese Patent Application Publication No. Sho 61-195951 have been proposed. In the technique disclosed in the former of these publications, V and Cr (WC grain growth inhibitors) are added simultaneously to a WC-Co cemented carbide (or WC-Ni cemented carbide) with the aim of suppressing the growth of the WC grains, and a cemented carbide having excellent wear resistance and toughness and having a dispersed phase of fine WC grains which have an average grain size of 0.7 .mu.m or less, can be obtained. In the latter of these publications, there is disclosed a tough cemented carbide having a Rockwell hardness (HRC) of 91 or more and a transverse rapture strength of 350 kg/mm.sup.2 or more which is achieved by the addition of VC, ZrN. In addition to V and Cr, Ta, Mo etc. are also known as elements for WC grain growth inhibitors (for example, see "Powders and Powder Metallurgy" 19(1972) p. 67).
On the other hand, with respect to hard carbons such as diamond, non-crystalline diamond etc., there have been advances in the development of the application thereof to cutting tools, wear-resistant parts due to their extremely high hardness and high heat conductivity. In particular, there have been vigorous advances in the development of hard carbon coated cemented carbide tools comprising a base material made of an ultra-fine grade cemented carbide having excellent toughness and a coating of hard carbon formed thereon by chemical vapor deposition.
However, there exists the following problems with the cemented carbides employed hereto as the base material for tools. The cemented carbides include about 3 to 20% of iron group elements such as Co, Ni etc., and the carbon dissolves into the binder phase during synthesis of the hard carbon coating whereby a hard film cannot be formed, or even if a hard film is partially formed, the adhesion thereof to the cemented carbide base material is not sufficient whereby the coating easily becomes peeled from the base material.
With a view to improving these problems in the prior art, the kind of technique disclosed in for example Japanese Patent Application Publication No. Hei 7-11375 has been proposed. In this technique, in order to obtain a product exhibiting excellent adhesion when the hard carbon coating is formed, there is adopted as the base material a cemented carbide in which substantially only WC grains or only WC particles and components other than binder phase comprising iron group elements are exposed on the surface, i.e. in which no iron group elements exist in the surface layer, and which fulfills at least one of the following conditions:
(a) the average grain size of WC in the surface layer is larger than the grain size in the interior; PA1 (b) the surface hardness is higher than the hardness of the interior; A hard carbon coating is then coated onto the surface of this base material.
However, even this technology cannot be said to sufficiently display those properties required to meet the recent advances with respect to the high density and multiple layering of printed circuit boards, and there is also the problem that the drill breaks during drilling. This problem is particularly striking when a fine diameter drill having a cutting point diameter of .phi.0.5 mm or less is used.